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JavaScript & importmap


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Remove the <img> tag, so we can see our normal page. Don't worry about our little penguin guy: we still have him up here in the logo.

When we refresh the page, notice that we do have a console.log() message... which says it's coming from assets/app.js. If we head over to assets/app.js... yup! There it is!

8 lines | assets/app.js
// ... lines 1 - 6
console.log('This log comes from assets/app.js - welcome to AssetMapper! 🎉')

How assets/app.js is Loaded

We know that we can write modern ES6 code in here, as well as import other files. We're going to do all of that. But first: How and why is this file even being executed? Our CSS is being loaded thanks to this nice, boring <link> tag. We don't see a <script> tag for app.js... but we do see this importmap() function. And that's the key.

Back over on the site, View the page source. Down here... this is what importmap adds. We're going to talk about each part, but the most important thing right now is at the bottom:

<script type="module">import 'app';</script>

Earlier, when we created an app.js file inside the public/ directory, this is almost exactly the code we wrote to load it. We used import and then the path to that file. But... this time, it just says app. Shouldn't it say something like /assets/app.12345.js"? How does it know that app refers to the final version of this file? This is where the importmap part, up here, shines.

The Wonderful importmap

This section is generated from an importmap.php file inside our project. The file isn't super-interesting yet: it'll be more useful soon when we talk about third party JavaScript. But it does have this app key that points to our assets/app.js file using its logical path.

22 lines | importmap.php
// ... lines 1 - 15
return [
'app' => [
'path' => 'app.js',
'preload' => true,

Thanks to that, this <script type="importmap"> dumps onto the page. When you import something that doesn't start with a ".", "/", or "../", that's called a bare import. We usually see this for third-party libraries. In the browser environment, when it sees a "bare import", your browser looks for an importmap on the page to find a matching entry. Our browser sees import 'app', finds this key here, and that's the path it downloads. It effectively copies this path here and pastes it down there. That's why our app.js file is being executed: it's team work between the importmap and the extra <script type="module"> that bootstraps our app!

The greatest thing about importmap is that it's not a Symfony thing: it's just an internet thing. It's how your browser works. We do have this importmap.php file, which is a Symfony thing. But once this is on the page, your browser is the star.

The importmap shim + Older Browsers

And importmap works in... most browsers. If you go to "" and search for "importmap"... it currently works in about 81% of browsers. That would be a huge problem, except that the importmap() function also dumps a shim. You can see that here. Thanks to this, if a browser doesn't support importmap, this adds that functionality. So, it's just going to work.

Importing Relative JavaScript Files

Head into app.js: let's write some modern code. In assets/, first create a new directory called lib/. And inside that, a new file called vinyl.js. You can organize things however you want, and this is one example of isolating some code into its own file.

I'll paste in the same class we had earlier. Back over in app.js, import that: import Vinyl and I can hit "tab" to autocomplete the from './lib/vinyl' part. Instantiate this using the same code as before... and then console.log(mix.describe()).

12 lines | assets/lib/vinyl.js
export default class {
constructor(title, year) {
this.title = title;
this.year = year;
describe() {
return `${this.title} was released in ${this.year}`;
// ... lines 11 - 12

5 lines | assets/app.js
import Vinyl from './lib/vinyl';
const mix = new Vinyl('Awesome Mix Vol. 1', 2014);

Using .js when Importing

I love it! We're coding like normal and using ./ to import. But when we go over and refresh... it doesn't work. Check out the 404: /assets/lib/vinyl coming from app.js.

So... what's going on here? We'll talk more later about debugging, but here's a hint: if you ever notice that your browser is trying to download a path that doesn't include the "version" part in the filename, something is wrong with your path... and you should check for typos.

Our problem is that we need to add the .js. It turns out that leaving the .js off is a Node thing... and it works if you're programming in Node. But in true JavaScript environments, like in your browser, you do need to include it.

5 lines | assets/app.js
import Vinyl from './lib/vinyl.js';
// ... lines 2 - 5

If we refresh now... that was it! It was really my editor's fault that the .js was missing when it autocompleted it. Fortunately, we can fix that! Go into your PhpStorm settings and search for "use file extension". Under "Code Style" and "JavaScript", change "Use file extension" to "Always".

This time... if we say import Vinyl and hit "tab", nice! We get the .js.

Automatic Importmap Entries

But the fun doesn't stop: there's something interesting happening behind the scenes. Click into this console.log()... just as an easy way to see the source of the final app.js file.

Yup, its contents look exactly like the original file, including the import from './lib/vinyl.js'. There's just one problem: that's not the final filename for vinyl.js!

Pop over to the Network tools, select "JS", and search for "vinyl". All files served by AssetMapper have a versioned part of their file name, and we see that for vinyl.js. But then... how the heck does our browser read ./lib/vinyl.js and know that it should download this long filename?

The answer, if you view the page source, is... dramatic drumroll... the importmap. And I love this. The importmap is constructed from two sources. The first source is obvious: importmap.php. And we'll add more entries to it soon. The second source is more subtle. Whenever our JavaScript imports another JavaScript file using a relative path, that imported file is automatically added.

This is powerful. It means that our final code can look like it originally does: ./lib/vinyl.js. But thanks to the importmap, our browser will smartly download the real file with the long version part in the name. This is really an internal detail, but it's cool to see how it works.

Okay, we've talked about importmaps a little... but we haven't seen its biggest superpower: using third party packages. Let's explore that next.